Internet and intranet information dispersal mechanisms (e.g., a multicast protocol) for event systems, such as a publish/subscribe system, and for push information distribution have recently received much attention. However, little work has been done on the inverse problem; i.e., collecting responses to a common question from a large number of distributed computing clients.
In a wide-area network, such as the internet or a corporate intranet, networks tend to be organized as a set of routers connected by links resulting in a graph where at least one path exists from any client of the network to any other client in the network. Routers receive messages along links and resend the messages along other links, as determined by the nature of each message, i.e., usually a destination network address stored in the message. The users of the network, that is, those entities such as computers, people, telecommunication devices, etc., who are using the network for communication between themselves, are all connected through at least one link to at least one router. For purposes of the description provided herein, however, both users and routers can be thought of as nodes, with users being distinguished as providing function other than only networking and routing.
A popular mechanism for multicasting a message in such a network is to distribute a message along a spanning tree of the network graph. That is, from the sending client, a set of links is chosen so that when the message follows these links from node to node, each router is visited exactly once and thus the message is delivered efficiently to each client. A different spanning tree may result for each sending client since the sending client determines the root of the multicast tree. In addition, the spanning tree may vary even for a single sending client. Most wide-area networks have multiple paths between clients allowing a choice as to which path is the best one to incorporate into a multicast spanning tree.